1. Field of the Invention
The present invention relates to a glass antenna attached to a vehicle side window glass such as for automobiles, particularly of the kind suited for receiving FM radio broadcast waves and AM radio broadcast waves.
2. Description of the Related Art
Examples of glass antennas having been heretofore known and put into practical use are an antenna attached to a marginal area or space of a rear window glass above or below a defogger electric heating element and an antenna including a conductive strip provided to a windshield, the former antenna however being not always capable of attaining a sufficient reception gain since the space occupied by the antenna is required to be small for eliminating the possibility that the antenna makes poor the visibility.
Further, the latter antenna, i.e., the antenna provided to a windshield can attain a relatively high reception gain but is encountered by a larger restriction on its attaching place than the rear window glass so that it never blocks the driver's visibility.
An antenna provided to a vehicle side window glass has come to attract attention since it is not encountered by the above described restriction and an automobile with a large side window such as a recreational vehicle (RV) has come to be used widely, and a number of proposal relating to such an antenna have been made as is disclosed in Japanese utility model provisional publication No. 58-61509 and Japanese patent provisional publication No. 61-265904.
However, most of the proposals relate to an antenna for receiving television (TV) broadcast waves, and there have been little proposals relating to an antenna for receiving FM (Frequency-Modulated) radio broadcast waves and AM (Amplitude Modulation) radio broadcast waves.
Particularly, in case of recreational vehicles (RV), many of their rear windows are capable of opening and closing, so the rear window glass is small-sized, resulting in a small marginal area or space above or below the defogger heater element for disposition of an antenna, and furthermore the rear windows are disposed nearly vertical, though this similarly occurs even in case their rear windows are of the type incapable of opening and closing. Thus, antenna strips arranged at such a small and nearly vertical marginal space of the rear window glass cannot attain a sufficient reception gain. This is particularly pronounced in case of antenna strips for receiving AM radio broadcast waves which are of low frequency and FM radio broadcast waves. Thus, an antenna provided to a side window glass for receiving radio broadcast waves has been demanded recently, but there have been little proposals to such an antenna.
Furthermore, for receiving ultrashort waves such as FM radio broadcast waves, it is a usual practice to dispose a plurality of antennas in predetermined places and switch the connections of the antennas selectively so that the ultrashort waves are received by one of the antennas which can attain a higher reception gain than any other antennas. In such diversity reception, even such an antenna which effects a particular drop or fall of gain (i.e., a so-called dip) in a particular direction, i.e., which exhibits a sharp directivity when used alone can be used. However, such an antenna cannot be used alone for reception of FM radio broadcast waves other than for diversity reception.